Multiqubit quantum entanglement is a central physical resource on which the non-classical computational power of quantum information technology is based. As a result of this, known quantum information processing methods with the potential to achieve substantial performance improvement over classical techniques are built on methods for producing and exploiting large-scale quantum entanglement. The two most well-known quantum-processing paradigms are: digital quantum computing, which is expected to provide exponential performance enhancement most notably for problems in cryptography (Shor's algorithm) and quantum simulation of chemical and biological molecules (quantum phase estimation algorithm); and quantum annealing, where engineered quantum fluctuations may provide qualitative enhancement in the heuristic sampling of classical optimization problems.
In both of these quantum-processing paradigms, the machinery for construction and maintenance of large-scale quantum entanglement relies on pairwise interactions between qubits. Larger-scale entanglement is then built up by combining many of these pairwise interactions, either by applying them successively in time in a pulsed manner, or by engineering many static pairwise interactions simultaneously to obtain an effective higher-order interaction perturbatively. This is largely limited by physical hardware constraints: two-qubit interactions are the only physical-level entangling operations that have yet been demonstrated in any qubit modality.